When manufacturing objects of ceramic materials by sintering together powder while using isostatic pressing, the powder is normally preformed into a manageable powder body. For the preforming there may be used, inter alia, conventional technique for the manufacture of ceramic goods, such as, e.g. injection moulding. The powdered ceramic material is then usually mixed, prior to the forming, with a temporary binder, for example a wax. After the preforming, the binder is driven off by heating, usually in vacuum, so that the preformed powder body is, in all essentials, free from binder before it is subjected to further treatment.
When the preformed body is subjected to the isostatic pressing at the sintering temperature, it must, in order to give a desired dense, sintered product, be enclosed in a casing or embedding material which, during the pressing, is able to prevent the pressure medium used, normally a gas, from penetrating into the powder body. The embedding material as well as its contents are freed from nondesirable gases during some process stage before the sealing. A known gas-permeable embedding material consists of particles of glass arranged as a layer on the preformed body, another known embedding material consists of particles of glass arranged as a mass, into which the preformed body is embedded. Such embedding materials of glass are made gas-impermeable by melting the glass. They are described, inter alia, in Swedish patent specifications 7600919-0 (publication No. 396 371) and 7813018-4 (publication No. 414 921).
After driving off the temporary binder from the bodies which have been formed from the powdered material by heating and vacuum treatment, it has proved that the preformed bodies obtained contain a carbonized residue of a few tenths of a per cent. In several ceramic materials, inter alia in silicon nitride, this carbonized residue may reduce the strength of the objects obtained by isostatic pressing of the preformed bodies. This, of course, entails a considerable disadvantage when it comes to objects in which the strength is of critical importance, such as in radial and axial turbines.
Removing the carbonized residue by oxidation at a temperature required for oxidation has proved to entail a drastic deterioration of the strength of the preformed body, so that this becomes insufficient for encapsulation of the body in an embedding material of glass. Resorting to a sintering of the body in order to improve the strength has proved to entail detrimental deformations of the objects manufactured during the isostatic pressing, especially in parts of the objects having thin and projecting portions, such as turbine wheel blades. Furthermore, the presintering has resulted in cracks occurring to a greater extent in the preformed bodies, which has led to increased scrapping.